Defects in final products, such as internal defects (detectable by ultrasonic testing) and surface defects such as blisters and sliver defects are often found in the rolled final product. Such defects are caused by trapping and accumulating nonmetallic inclusions, mold powders and bubbles in the cast products when molten magnetic metal, particularly steel is continuously cast in a curved continuous casting machine.
Prior art attempts to prevent these defects include the following:
1. Cleaning up the molten metal by using various ladle refining processes. PA1 2. Preventing reoxidization of the molten metal by fastening the seals of the tundish. PA1 3. Superheating the molten metal and causing the inclusions to float up in the mold to mold powders at the meniscus which results in removal of the inclusions from the molten metal. PA1 4. Preventing the particles of the ladle slag and the tundish powders from being trapped into the cast products by using a large volume tundish. PA1 5. Installing a vertical bending machine to float up the inclusions, and absorbing them into the molten mold powders at the meniscus. PA1 6. Preventing inclusions and mold powders from being trapped in the cast products by reforming the immersion nozzle profile. PA1 7. Trapping inclusions and mold powders with trapping boards installed at the outlet of the immersion nozzle ports. PA1 8. Preventing the jet streams of the molten metal from penetrating into the molten metal pool in the slab by installing reflecting boards at the outlets of the immersion nozzle ports.
However, these prior art procedures have not been found to be sufficient to clean the molten metal in actual plant manufacturing processes which are required to meet targeted high quality levels.
Inclusions, mold powders and bubbles which are introduced into the molds of continuous casting machines are trapped and accumulated in the cast products when the throughput speed of the molten metal exceeds a definite value. It is typically not possible to remove them by floating them up to the molten mold powders on the meniscus when throughput speeds exceed the definite value.
It was also common practice to attempt to control the jet streams of the molten metal ejected from the immersion nozzles by optimizing the profiles of the outlet ports of the immersion nozzle or by reducing the casting speed. But these attempts were not sufficient to prevent defects caused by trapping or accumulating inclusions or mold powders introduced into the molten metal.
An electromagnetic brake (EMBR) system was proposed to cope with these problems as reported in Iron Steel Eng. May 1984 p.41-p.47, J. Nagai, K. Suzuki, S. Kozima and S. Kallberg, and also in U.S. Pat. No. 4,495,984. The braking force was obtained by introducing static magnetic fields perpendicular to the flow direction of the molten metal jets from the immersion nozzle. The difference in speed between the molten metal in the jets and the rest of the mold created a voltage and thus created eddy currents. These eddy currents interacted with the static magnetic field, creating a braking force (Lorentz force), which acted in a direction of opposed to the metal flow.
The attempted effects of the EMBR system were reducing the flow velocity of the molten metal in the mold, preventing trapping and accumulating mold powders and inclusions into the cast products and floating the inclusions introduced into the molten metal. Under certain conditions the system reduced the internal defects (detectable by ultrasonic testing) of the final products caused by the mold powders, and reduced the trapping and accumulating inclusions in the upper half of the strands in the curved mold casters. It was believed that increasing the flow velocity of the molten metal jet from the nozzle would provide a more effective braking effect than other methods because the braking effect of the Lorentz force was proportional to the jet stream speed.
However, under commercial casting conditions it was often experienced that the effects of the EMBR system were not enough and that the EMBR system actually damaged the quality of the cast products, especially in high speed casting.
According to U.S. Pat. No. 4,495,984, the flow direction of the jet streams of the molten metal can be changed by the EMBR system as though the streams had collided against a wall, but it is in fact impossible to obtain uniform flow by splitting the energy of the jet streams, and the jet streams tend to be diverted toward a direction where the static magnetic field is not in effect.
Many ideas directed to the arrangement of the iron cores were proposed to optimize the static magnetic field in the continuous casting mold.
Japanese patent Kokai 59-76647 disclosed the idea of reducing the speed of the molten steel and splitting and stirring the streams of the molten steel by forming a static magnetic field just below a continuous casting mold.
Japanese patent Kokai 62-254955 disclosed various sizes and arrangements of the iron cores in a continuous casting mold.
Japanese patent Kokai 63-154246 disclosed the idea of arranging the magnetic poles at the meniscus and/or the bottom of a continuous casting mold.
However these prior art processes were defective and caused inclusions to accumulate deeply in the cast products when the casting conditions (such as casting speed, dimensions of the cast products, profile of the immersion nozzle and the level position of the meniscus) were changed and differed from definite optimum conditions.
In other words, these prior art processes were able to brake the streams of molten metal only under certain specific conditions, but once the casting conditions were changed, the beneficial effects of the EMBR system were reduced or sometimes the EMBR system even degraded the quality of the cast products.